This application is based on Patent Application No. 10-306192 filed on Oct. 27, 1998 in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to an ink jet printing apparatus and an ink jet printing method and particularly, to an ink jet printing apparatus and an ink jet printing method using a print head in which a plurality of electro-thermal transforming elements are provided for each ink ejection opening and are adapted to be independently driven.
2. Description of the Prior Art
There have been provided an ink jet print head in which a plurality of electro-thermal transforming elements are provided in an ink passage communicating with an ink ejection opening and are individually driven, and a printing apparatus which uses such a print head, by the assignee of the present application. Such a print head has, for example, two independently driveable electro-thermal transforming elements in each of ink passages and generates bubbles in ink by utilizing thermal energy caused by driving the electro-thermal transforming elements so as to eject the ink by the pressure of the bubbles. Upon printing a low-density part of an image, one of the two electro-thermal transforming elements is driven to eject an ink droplet for forming the low-density part of an image. For a high-density part of the image, the two electro-thermal transforming elements are driven to eject an ink droplet for forming the high-density part of the image. Thus, printing having more gradation levels is performed so that images of photograph quality can be formed. Further, since the printing apparatus can form the image with the ink droplet ejected by driving these two electro-thermal transforming elements when forming the image such as a character, such an image can be formed with relatively large ink dots. As a result, printing can be performed at a resolution similar to that of printing that is originally performed at a low resolution; thereby, high speed printing can be accomplished.
In connection with a configuration having the plurality of electro-thermal transforming elements in each ink passage as described above, it has been disclosed in Japanese Patent Application Laid-open No. 55-132259 (1980) that gradation printing can be performed by varying generation timings of respective bubbles by the driving of the respective two electro-thermal transforming elements in the ink passage and changing amount of the respective ejected ink droplets.
Driving means which drives two electrothermal elements with a time lag (delay) of a slight amount for stable ejection of ink has been also disclosed in Japanese Patent Application Laid-open No. 10-071718 (1998) by the inventors of the present application.
However, in the ink jet printing apparatus of serial scan type in which printing (printing scan) is performed by reciprocating the print head relative to a printing medium and in which the plurality of electro-thermal transforming elements are driven for ejection of ink as described above, the following problems may occur, for example, when the apparatus has a plurality of ink ejection modes which have different ejection amounts of ink from each other.
In order to obtain an appropriate landing accuracy of the ejected ink in the ink ejection mode of small ejection amount, it is desirable that an ejection speed is faster than a predetermined value, for example, approximately 8 m/sec. As a configuration to achieve this ejection speed, it may be considered that a cross section of the ejection opening is made smaller. On the other hand, for the print head having such an ejection opening of smaller cross section, when all of the plurality of electro-thermal transforming elements provided for one ejection opening are simultaneously driven, a relatively large ejection speed, for example, approximately 18 m/sec, can be achieved. Furthermore, in this case, a flying ink droplet can take a shape as shown in FIG. 1 for the case of xe2x80x9csimultaneous heatxe2x80x9d. More specifically, when the plurality of electro-thermal transforming elements are simultaneously driven, both a main droplet section of the ejected ink and a tailing section ejected in a trailed manner by the main droplet fly at noticeably large speeds and a difference between these speeds becomes noticeably large. The serial scan type printing apparatus, which scans the print head ejecting such ink droplet to the printing medium to perform printing, forms ink dots on the printing medium as shown in, for example, FIG. 2A. In FIGS. 2A through 2D, an arrow denotes a scanning direction of the print head, and a large dot is formed with the main drop section of the ejected ink and a small dot is formed with the tailing section of the ejected ink.
A reason why the dot from the main drop section and that from the tailing section are formed in a deviation manner from each other will be described below with reference to FIG. 3. FIG. 3 is a diagram showing the respective ejection of the main drop section and the tailing section from the print head and their landing positions on the printing medium. In this figure, the print head moves in the direction shown by an arrow Vcr at a speed Vcr. A distance between the printing medium and the ejection opening of the print head is denoted as d. Also, the ejection speed of the main drop section is denoted as Vm, the ejection angle with respect to the vertical direction of the printing medium is denoted as xcex8, and a distance in a head moving direction from the ejection start position of the main drop section to the landing position on the printing medium is denoted as Dm. In this case, the ejection angle of the tailing section is also denoted as xcex8, but the distance in the head moving direction from the ejection start position to the landing position is denoted as Dt and the ejection speed is denoted as Vt for the tailing section. Further, when time between the ejection of the main drop section from the ejection opening and the ejection of the tailing section from the ejection opening is denoted as xcex94Td, the distance xcex94Dd over which the print head moves during the time xcex94Td is expressed as follows:
xcex94Dd=Vcrxc3x97xcex94Tdxe2x80x83xe2x80x83(1)
From the foregoing, time Tm between the ejection start of the main drop section and its landing is expressed as follows:
Tm=d/(Vmxc3x97cos xcex8)xe2x80x83xe2x80x83(2)
Also, the distance Dm is expressed as follows:
Dm=(Vcr+Vmxc3x97sin xcex8)xc3x97Tmxe2x80x83xe2x80x83(3)
From the equations (2) and (3), the following equation is obtained:
Dm=dxc3x97(Vcr/Vm+sin xcex8)/cos xcex8xe2x80x83xe2x80x83(4)
Similarly, the distance Dt in the head moving direction from the ejection start position of the tailing section to the landing position is expressed as follows:
Dt=dxc3x97(Vcr/Vt+sin xcex8)/cos xcex8xe2x80x83xe2x80x83(5)
From the foregoing, the distance xcex94D between the landing points of the main drop section and the tailing section is expressed as follows:
xcex94D=Dt+(xcex94Ddxe2x88x92Dm)xe2x80x83xe2x80x83(6)
From the equations (1), (4), (5), and (6), the following equation is obtained:
xcex94D=Vcrxc3x97{(d/Vtxe2x88x92d/Vm)/cos xcex8+xcex94Td}xe2x80x83xe2x80x83(7)
From this equation, the following can be seen.
At the higher speed the print head moves, the larger the distance xcex94D becomes. The term (d/Vtxe2x88x92d/Vm) in the equation indicates that, when the larger the difference between the respective flying times of the main drop section and the tailing section is, the larger the distance xcex94D becomes. That is, when the larger the difference between the respective ejection speeds of the main drop section and the tailing section is, the larger the distance xcex94D becomes. Further, when the larger the distance d between the printing medium and the ejection opening of the print head is, the larger the distance xcex94D becomes.
When the ejection angle xcex8 meets the condition: |xcex8| less than 15 [deg], any possible effects of the ejection direction may be usually ignored because 1/cos xcex8 is what has a value of (1 less than ) 1/cos xcex8 less than 1.04. An example of measurements taken by the present inventors for the plurality of electro-thermal transforming elements (hereinafter simply referred to as a xe2x80x9cheaterxe2x80x9d) driven simultaneously are as follows:
Vm=18 m/sec
Vt=8 m/sec
xcex94Td=28 xcexcsec
xcex8=15 deg
In this case, when the print head is moved at the speed of Vcr=0.508 m/sec and the distance between the printing medium and the ejection opening of the print head is d=1.9 mm, the following distance xcex94D is obtained:
xcex94D=81 xcexcm.
When performing printing at a dot density of 360 dpi, the print head is designed so that the dot diameter becomes approximately 100 xcexcm. The print head used for the measurement example described above also forms dots of approximately 100 xcexcm across. That is, the dot radius is 50 xcexcm, and therefore, when the distance xcex94D between the landing points of the main drop section and the tailing section is 81 xcexcm, the dot from the tailing section is formed outside another dot from the main drop section.
When a printing image is formed with dots having such a shape, the dots from the tailing sections are formed ahead of a line image of the main dots in the scanning direction of the print head (the arrow in the figure) as shown in FIG. 4 and the resulting image may have a front edge which appears blurred as compared with a rear edge of the printing image in the scanning direction. As a result, the print quality may degrade.
It is an object of the present invention to provide an ink jet printing apparatus and an ink jet printing method which can prevent any degradation in print quality from being caused by dots formed of tailing sections of an ejected ink.
In a first aspect of the present invention, there is provided an ink jet printing apparatus for using a print head which has a plurality of electro-thermal transforming elements provided for one nozzle and generates a bubble by utilizing thermal energy caused by driving the plurality of electro-thermal transforming elements to eject ink and for performing printing onto a printing medium by ejecting the ink from the print head, the apparatus comprising:
scanning means for scanning the print head relative to the printing medium; and
driving means for driving the plurality of electro-thermal transforming elements of the print head to eject ink from the nozzle while the scanning means scans the print head, the plurality of electro-thermal elements being driven at different timings to eject the ink so as to form a dot more approximate circular shape than that formed with the ink ejected by driving the plurality of electro-thermal transforming elements simultaneously.
In a second aspect of the present invention, there is provided an ink jet printing method for using a print head which has a plurality of electro-thermal transforming elements provided for one nozzle and generates a bubble by utilizing thermal energy caused by driving the plurality of electro-thermal transforming elements to eject ink and for performing printing onto a printing medium by ejecting the ink from the print head, the method comprising the steps of:
scanning the print head relative to the printing medium; and
driving the plurality of electro-thermal transforming elements of the print head to eject ink from the nozzle while the scanning step scans the print head, the plurality of electro-thermal elements being driven at different timings to eject the ink so as to form a dot more approximate circular shape than that formed with the ink ejected by driving the plurality of electro-thermal transforming elements simultaneously.
According to the configuration described above, a plurality of electro-thermal transforming elements provided for one ejection opening are driven at different timings from each other so that a dot formed with an ink ejected by driving them may be more approximate to a circular shape than a dot formed by driving them simultaneously. More specifically, when an ink droplet is separated into a main drop section and a tailing section through such an ink ejection caused by driving the plurality of electro-thermal transforming elements with a time lag, the dot formed from the tailing section can be made closer to another dot formed from the main drop section on the printing medium and the combined dots are more approximate a circular shape as a whole.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.